1. Learning Objectives
You should learn :
About the different types of variables,
How to identify them when doing your practical work.

2. Variables are things that vary and change

3. Variables In any experiment there are 3 variables:
an independent (or input) variable
a dependent (or outcome) variable
some control variables
Let’s look at each type….

4. Independent (input) variable
This is the thing that you decide to change.
Example 1
Investigating how a weight affects the length of an elastic band.
You decide the weight to apply, so:
Weight is the independent variable.

5. Independent (input) variable
This is the thing that you decide to change.
Example 2
Investigating how the rate of cooling of a beaker depends on the initial temperature.
You decide the initial temperature, so:
initial temperature is the independent variable.

6. Independent (input) variable
This is the thing that you decide to change.
Example 3
Investigating how the current through a resistor depends on the voltage across it.
Which is the independent variable as you add cells?
The voltage is the independent variable.

7. Dependent (outcome) variable
This is the variable that changes as a result. It is the variable that you measure.
Example 1
Investigating how a weight affects the length of an elastic band.
You measure the resulting length of the elastic band, so:
Length is the dependent variable.

8. Dependent (outcome) variable
This is the variable that changes as a result. It is the variable that you measure.
Example 2
Investigating how the rate of cooling of a beaker depends on the initial temperature.
You measure the temperature every minute as it cools, so:
temperature is the dependent variable.

9. Dependent (outcome) variable
This is the variable that changes as a result. It is the variable that you measure.
Example 3
Investigating how the current through a resistor depends on the voltage across it.
Which is the dependent variable here?
The current is the dependent variable.

10. Control variables
These are all the variables that must not change, to make sure it is a fair test.
Example 1
Investigating how a weight affects the length of an elastic band.
You must use the same elastic band all the time, and the same scale etc, so it is a fair test.

11. Control variables
These are all the variables that must not change, to make sure it is a fair test.
Example 2
Investigating how the rate of cooling of a beaker depends on the initial temperature.
You must use the same beaker, with the same amount of water, in the same position in the room, at the same room temperature, so it is a fair test.

12. Use the same circuit at the same temperature each time.
Control variables
These are all the variables that must not change, to make sure it is a fair test.
Example 3
Investigating how the current through a resistor depends on the voltage across it.
Which are the control variables here?
Use the same circuit at the same temperature each time.

13. Tables When you draw up a table of your results,
the independent variable goes in the first column, like this:
If you take several readings of the dependent variable, then you can calculate the mean (average)
Then your results will be more reliable.

14. same elastic band, same scale, etc, so it is a fair test.
In Summary
Example 1
The independent variable is ?
weight
The dependent variable is ?
length of the elastic
The control variables are ?
same elastic band, same scale, etc, so it is a fair test.

15. temperature as it cools
In Summary
Example 2
The independent variable is ?
initial temperature
The dependent variable is ?
temperature as it cools
The control variables are ?
the same beaker, with the same amount of water, in the same position in the room, at the same room temperature, so it is a fair test.

16. In Summary voltage (p.d.) current
Example 3
The independent variable is ?
voltage (p.d.)
The dependent variable is ?
current
The control variables are ?
the same circuit, at the same temperature each time, so it is a fair test.

17. Learning Outcomes
You should now:
Understand the difference between - independent, - dependent, and - control variables
Be able to identify these variables when doing your practical work.